Photovoltaic (PV) modules are used to generate electricity from sunlight by the photovoltaic effect. It has been recognized for decades that if these modules could be mass produced at low cost, they could be used to meet a considerable portion of the world's energy needs. Major companies, such as Royal Dutch/Shell and BP-Amoco, have stated that PV modules have the potential to become a major energy source and that their use has significant benefits to the global environment. However, for these benefits to be realized, PV modules must be produced at many times the current volume and at costs below $100/m2, as discussed by Bonnet et. al. in “Cadmium-telluride material for thin film solar cells”, J. Mater. Res., Vol. 13, No. 10 (1998). Currently, PV modules are manufactured in small quantities at costs of about $500/m2. About one hundred times the current yearly production is required to sustain a PV module manufacturing capacity that can contribute just 5% of the current electricity generated. Consequently, the manufacturing volume of PV modules needs to be greatly increased and costs significantly reduced.
To realize the required increases in production volume and decreases in manufacturing costs, PV modules must be produced as a commodity. Commodity level manufacturing requires innovation to develop highly automated production processes and equipment, which are designed to specifically fabricate the commodity product. Commodity manufacturing necessitates high production speeds (high throughput), minimal labor costs, and a continuous process flow. Low capital costs and ease of expanding production capacity also facilitate commodity manufacturing. There are a variety of known PV devices, but only the cadmium telluride (CdTe) thin film PV device has the potential to satisfy the requirements for commodity manufacturing.
Since 1974, there have been many industrial efforts to create technologies for CdTe PV module manufacturing. Most of these industrial efforts, as exemplified by the teachings of U.S. Pat. Nos. 4,319,069, 4,734,381, and 5,501,744, have been terminated because of fundamental inadequacies in their manufacturing technologies. To date, no technology suitable for commodity level manufacturing of CdTe PV modules has been developed, thus demonstrating the need for innovation in this area.
The most common CdTe PV cells are thin film polycrystalline devices, in which the CdTe layer is paired with a cadmium sulfide (CdS) layer to form a heterojunction. The thin films of a CdS/CdTe PV device can be produced through a variety of vacuum and non-vacuum processes. Of the many types of thin film deposition methods, sublimation in vacuum is most amenable to commodity manufacturing. This is because vacuum sublimation of CdS/CdTe PV modules exhibits deposition rates 10 to 100 times higher than any other PV module deposition method. Vacuum sublimation of the semiconductor layers for CdS/CdTe PV modules can also be performed in modest vacuum levels and does not require costly high vacuum equipment. Vacuum deposition methods for other thin film PV devices require costly, complex high vacuum equipment and results in low throughput.
Due to the high rate of deposition and low capital cost, the CdS/CdTe thin film cell fabricated by vacuum sublimation is the most suitable for commodity level manufacturing of PV modules. However, cadmium is a Group B carcinogen. According to U.S. government regulations, the quantity of this material which can be lawfully released into the environment or into an occupational setting is extremely small. The known prior art in CdS/CdTe vacuum sublimation requires process and hardware innovations to achieve occupational and environmental safety as required by federal regulations, as well as commodity scale manufacturing.
One known configuration for a CdTe device is the back wall configuration, in which the thin films are deposited onto a glass superstrate, hereinafter referred to as a substrate. The CdTe device is most often fabricated on a glass substrate coated with a transparent conductive oxide (TCO) film onto which other film layers are deposited in the following order: a) a CdS film, b) a CdTe film, c) an ohmic contact layer, and d) a metal film. Along with the deposition of these films, many heat treatments are also needed to enhance the device properties. The TCO and the metal films form the front and back electrodes, respectively. The CdS layer (n-type) and the CdTe layer (p-type) form the p/n junction of the device. The cells are deployed with the substrate facing the sun. Photons travel through the glass and TCO film before reaching the p/n junction of the device. A module is formed by interconnecting individual cells in series to produce a useful voltage.
Thus, a process for manufacturing CdS/CdTe modules includes the following steps: 1) cleaning the TCO coated glass substrates, 2) heating the substrates, 3) depositing an n-type CdS layer, 4) depositing a p-type CdTe layer, 5) performing a CdCl2 treatment to improve CdTe grain structure and electrical properties, 6) forming a p+ ohmic low resistance contact layer to improve current collection from the CdTe, 7) depositing a metal layer (metallization) to form the back electrode, 8) scribing the film layers into individual cells, 9) interconnecting the cells in series and providing a means of electrical connection to the module, and 10) encapsulating the finished module.
All of the prior art methods for the production of CdTe modules have limitations that render them unsuitable for commodity level manufacturing. For example, prior art methods of CdCl2 treatment are disconnected, low throughput batch operations, rather than continuous flow processes. These batch type processes are inefficient and involve extremely high costs in order to increase throughput to the commodity manufacturing level. Most of the known methods of CdCl2 treatment also require rinsing, which generates liquid wastes that contain cadmium. Known methods of ohmic contact formation are also batch type processes that exhibit low throughput rates. Prior art metallization steps also exhibit low throughput and require costly process equipment. It is necessary to improve the current methods of CdCl2 treatment, ohmic contact formation, and metallization in order to achieve high throughput continuous processes.
Prior art methods for scribing the layers to form a module include laser scribing, mechanical scribing, and abrasive blasting. Known laser scribing methods used in the PV industry are associated with low production speed and high capital cost. Laser scribing was abandoned recently in one industrial setting due to laser equipment failure as discussed by Borg in, “Commercial Production of Thin-Film CdTe Photovoltaic Modules”, NREL/SR-520-23733, October 1997. Known mechanical and abrasive blast scribing methods have only been shown on a small scale as typified by U.S. Pat. No. 5,501,744 to Albright and require innovation and improvement to be suitable for commodity level manufacturing.
Specific examples of prior art relating to CdS deposition and CdTe deposition performed by vacuum sublimation are described in detail below. The other prior art steps that are necessary to form a complete CdTe PV module are also discussed below.
One known vacuum method of producing CdTe solar cells by vacuum sublimation is taught in U.S. Pat. No. 5,536,333 to Foote et. al. This method is further described by Sasala et. al. in “Technology Support for Initiation of High-Throughput Processing of Thin-Film CdTe PV Modules”, NREL/SR-520-23542, pp. 1–2, (1997). These references discuss a technique known as vapor transport deposition (VTD), which involves heating of the semiconductor materials in a contained vessel in order to create vapor. An inert carrier gas, such as nitrogen, transports the vapor of the semiconductor to the substrate through heated conduits. The substrate is held horizontally in a heated environment and supported from beneath by ceramic rollers in the heated environment. The deposition of the semiconductor is made onto the top surface of the substrate. In accordance with this prior art method, the ceramic rollers prevent the glass substrate from sagging under its own weight due to the elevated temperatures involved.
The entire VTD method is very complex and costly. It is possible to deposit a complete CdTe solar cell in a very short time and at sufficiently low substrate temperatures to eliminate glass sagging completely or reduce it to a very small acceptable value. Thus, the expensive ceramic rollers of the VTD method are not needed. Reloading starting material may also be performed in a much simple manner than as shown in this prior art. Since the films are thin, only small amounts of material are required to form them. Consequently, only very small volumes of starting material are needed for many days of operation, thus eliminating the need for this complex reloading arrangement. The heated vessels of this method contain toxic vapors, which pose significant occupational safety problems when they are opened for reloading during processing. In the VTD method, vapors are transported through long distances in a carrier gas, an arrangement which will likely lead to the formation of very small nano-particles through condensation of the vapors. These nano-particles degrade the film qualities and lead to occupational hazards when the system is serviced. Furthermore, in the VTD method, the continuous flow of carrier gas has to be maintained along the substrate. Any CdS or CdTe vapors that are carried past the substrate will be wasted. Any deposits of waste material on the inner surface of the vacuum chamber, pumps, exhaust, etc. must be cleaned, thereby exposing maintenance workers to toxic materials and raising occupational safety issues. In order to prevent unwanted condensation of CdS and CdTe vapors, the VTD method also requires continued heating of large portions of the equipment, including the vaporization vessel itself, the conduits, the deposition chamber, etc. This wastes energy and increases the capital costs. The VTD method is only used for depositing the p/n junction layers. Other processing steps, such as the CdCl2 treatment, ohmic contact formation, and metallization are inherently low throughput batch processes. Scribing is taught to be either laser scribing or photolithography, both of which are slow and costly processes.
In another prior art reference entitled “The CdTe Thin Film Solar Cell,” International Journal of Solar Energy, vol. 12, 1992, Bonnet proposes an inline production method for fabricating CdS/CdTe layers using a close-spaced sublimation (CSS) type deposition process. This prior art reference describes inline deposition within one vacuum boundary for only the steps of substrate heating, CdS deposition, and CdTe deposition. The other steps of CdCl2 treatment, ohmic contact formation, and metallization are not shown as part of a continuous inline vacuum process, and are presumably performed by previously known methods. As described above, known methods for performing these steps have limitations. The Bonnet method does allow multiple substrate processing of more than one film. However, it is not clear if long term operation and uniformity of deposition over time and across large substrates are achieved. Furthermore, the Bonnet method relies on CSS, which by definition, requires a space of 2–3 mm between the source and the substrate. This gap will allow a vapor leak at the edge of the substrate. As the source charge sublimes away over time the gap will increase. It is the present inventors' experience that this gap and the associated vapor leak causes non-uniform deposition on the substrates and also results in condensation of toxic materials on unwanted areas on the inner surfaces of the process chamber. This leak will be reduced if the background chamber pressure is held high enough to decrease the mean free path between gas molecules in the process chamber. However, higher pressures lead to lower deposition rates and greater nano-particle formation. An operating pressure of 750 millitorr is specified. At these pressures, nano-particles will be formed, since the vapor will homogeneously condense in the ambient gas near the edge of the deposition space. These very small particles degrade film quality and pose a health hazard to workers during routine maintenance inside the vacuum chamber.
Each of the individual prior art process steps required to produce a CdTe PV module exhibits limitations, as outlined above. In addition, nothing in the prior art describes an overall process to perform the series of steps of substrate heating, depositing an n-type CdS layer, depositing a p-type CdTe layer, performing a CdCl2 treatment, and forming an ohmic contact inline, continuously, and in one vacuum boundary. In particular, the steps of CdCl2 treatment and ohmic contact formation require significant innovation before they can be included in a continuous inline vacuum process. Such a continuous inline vacuum process would have significant advantages for commodity manufacturing of CdTe PV modules.
Any vacuum process for manufacturing CdTe PV modules would also require an apparatus to transport substrates through the process steps within vacuum and to transport the substrates into and out of vacuum rapidly. This apparatus should be robust, simple, and low cost. The apparatuses described in the prior art simply do not meet these requirements.